How to simulate paint mixing?
Everybody who ever painted knows that blue and yellow make green. Yet on a computer (at least in the RGB model), blue and yellow make white. Yes, it's mixing paint vs adding light, but how do you explain the difference, and how can one model the "paint mixing" on a computer, e.g. in gimp?
Reading this sometime later I realized that this does not answer the question.
The answer is indeed blending modes.
And yes, blue and yellow makes black. But the problem with this is not the multiply mode, the problem is that the mix should be yellow and cyan, not blue.
In RGB mode yellow is a mixture of Red and Green, and if you add blue you get white.
In multiply mode it is let's say the opposite.
You take Yellow (pigment primary) and Blue (in pigment blue has cyan and some magenta) so basically if you keep mixing them, as you are using all 3 primary colors you get black.
Here is a topic where I try to address some pre-computer blending modes: https://photo.stackexchange.com/questions/83002/what-is-the-pre-computer-version-of-photoshops-blending-modes/83006#83006
The pigment samples
There are some applications that simulate paint styles.
One free and good one is MyPaint http://mypaint.intilinux.com/
One that has a great look, also free shipped in Windows 8 is FreshPaint http://apps.microsoft.com/windows/es-mx/app/fresh-paint/1926e0a0-5e41-48e1-ba68-be35f2266a03
The commercial leader is Corel Painter http://www.painterartist.com/us/product/paint-program/
The "wet" component
These programs have a component that will help you a lot. They all have a fresh paint mode to some degree. This is that mix colors like a wet medium (oil, acrylic, watercolor). Not only they mix the color by subtraction, but they drag it.
A no specialized software will put the next color above the previous one.
Using normal software
If you choose to use gimp you need to change the Paintbrush mode.
In the tool box choose Mode > Multiply. This will not look like an oily medium but as a marker. The blue marker will make a previous yellow color green, but also will make each pass darker, like when you use a water based marker.
Use RGB color mode
Don't use a CMYK model to "paint". The use of black for print use very specific matrices to assign the ink.
You will be making crazy mixings and these effects work better in RGB. You can transform the RGB to a CMYK output later.
The light samples
I probably would use a 3d render to simulate projections. As always my first free choice is Blender http://www.blender.org/